I want to install a timer switch (it has five wires, red (load), black (hot), white (common), green (ground) and yellow (three-way switching but not needed for my application) to replace one switch in a 2-gang switch box having six incoming romex connections (see first photo). One switch turns on outside lights, and it is this switch I want to replace with the timer. The other switch turns on inside lights and I want it to remain in the circuit. Both loads are on the same circuit - all grounds coming into the box are twisted together and all whites terminate in the same wire nut. There are additional downstream loads (interior lights and fans in a different room) which rely on the junction box connections but have their own downstream switches (this is the source of my confusion).
In the 2-gang junction box, the red (load) wire was shared between both switches (see second photo) - it was a single wire stripped in the middle and wrapped around the screw of the second switch then went on to a wire nut with two black wires I found go out to the respective switched loads - you can see this in photo 2. I hooked up the timer as set out in the instructions, but the downstream lights did not work at all and the timer switch did not switch the intended load (outside lights) which just stayed off.